With development of science and technology, head-up display (HUD) systems are used more and more widely in automobiles. A head-up display system in an automobile can display important driving information in real time, such as speed, engine revolution, fuel consumption, tire pressure, navigation and information from external smart devices, in view field of a driver on a front windshield. In this way, the driver can see the driving information without looking down, thus avoiding distracting attention to roads in the front; meanwhile, the driver do not have to adjust eyes when viewing faraway roads and nearby meters, thus avoiding eyestrain, greatly enhancing driving security and improving driving experience.
Currently, the head-up display technology is achieved mainly in two modes: luminescence imaging mode and projection imaging mode. The projection imaging mode uses the front windshield itself of the automobile or an extra disposed optical element to perform projection display, while adopting the front windshield to reflect a projected image is a mode having the simplest structure. An ordinary front windshield is generally laminated glass, which is formed by sandwiching a thermoplastic polymer film (such as polyvinyl butyral, i.e. PVB) between at least two glass substrates having a certain curvature. Light emitted by a projection light source of the head-up system may be reflected when it travels through two air-contacting surfaces of the laminated glass, and reflected images on the two surfaces may be shifted from each other to form double images (so called ghost) interfering mutually which are even more obvious especially if the laminated glass has a great thickness (generally, greater than 3 mm), thereby greatly limiting the definition of the projected images.
To solve the problem of double images of the head-up system on the front windshield of the automobile, several solutions have been proposed in prior art. For example, one of the solutions disclosed in CN101038349A, US2002172804A1 and US2007148472A1 is that a wedge-shaped polymer film is used as an intermediate film of the laminated glass, such that the laminated glass has a wedge-shaped cross-section decreasing from top to bottom, thus enabling the reflected images, seen by the driver, on the two surfaces to substantially overlap, and as a result greatly solving the problem of double images. Similar to the above, only a part of a glass substrate may have a wedge-shaped cross-section, as disclosed in U.S. Pat. No. 6,414,796B1. However, these technical solutions have following disadvantages: (1) the problem of double images may not be solved thoroughly, such that these solutions are not suitable for high-definition image display; (2) a PVB film of special specification should be adopted, and the price thereof is 7-10 times higher than that of an ordinary PVB film, the manufacturing process thereof has a high difficulty, such that the cost of the material and the manufacturing process is extremely high; (3) these solutions are easily affected by optical designs for automobile models, and generally the front windshield of a specific automobile mode should be redesigned.
Another solution is that an optical functional layer capable of changing the direction of polarized light or a reflective polarizer capable of reflecting p-polarized light or s-polarized light may be disposed on the surface of the laminated glass or in the laminated glass, and the p-polarized light or the s-polarized light emitted by the projection light source of the head-up display system is incident at a given angle (e.g. Brewster's angle), such that the reflected image on a certain surface may be eliminated as much as possible by means of the reflection property of the laminated glass for different polarized light, and as a result, double images are prevented. Such a technical solution is disclosed in patents such as EP0836108A2, EP0844507A1, U.S. Pat. No. 6,327,089B1, CN1969219A, U.S. Pat. No. 7,355,796B2, CN101151568A, U.S. Pat. No. 7,839,574B2, CN1732404A, CN102256910A, etc. However, the technical solution may be achieved only when the extra optical functional layer or the reflective polarizer is additionally disposed on/in a part of the laminated glass (i.e. head-up display projection area), thereby inevitably increasing the material cost and the manufacturing process difficulty. For example, the p-polarization reflective polarizer disclosed in CN1732404(A) has up to tens or hundreds of polymer layers; meanwhile, the uniformity and appearance of the entire front windshield may be destroyed; besides, the bonding strength on the part of the laminated glass where the optical functional layer or the reflective polarizer is disposed may be reduced to cause potential safety hazard; moreover, a part of the optical functional layers or the reflective polarizers have a relatively low transmissivity for visible light, which may affect the view field of the driver or the appearance of the front windshield.
Furthermore, in the prior art, the luminance of a reflected image formed on one of interfaces may be reduced by additionally disposing an anti-reflective (AR) coating on the surface of the laminated glass. For example, the technical solution disclosed in U.S. Pat. No. 7,864,431B2 is that the anti-reflective coating is disposed on an air-contacting surface of a laminated glass or a low-emissivity (low-E) coating is additionally disposed on the surface on which the anti-reflective coating is not disposed. However, this technical solution has following disadvantages: (1) the anti-reflective coating may not withstand harsh usage environment, such as acid rain, dust, wiper friction, alkaline cleaner, etc., when it is deposited on the air-contacting surface of the laminated glass, especially on the exterior surface of the laminated glass; (2) the anti-reflective coating has a relatively complicated structure, especially the anti-reflective coating which has fine broadband anti-reflection, a reflectivity of approximately zero, and a fine appearance both at a small angle and a large angle requires multilayer deposition, such that its total thickness is up to hundreds of nanometers, and therefore it is unsuitable for being deposited on the front windshield of the automobile over a large area; (3) the anti-reflective coating may not solve the problem of double images thoroughly, especially light purple or light blue reflected light may be still visible from a large angle; (4) the anti-reflective coating deposited on the exterior surface of the laminated glass may be covered by a water layer in rainy days, and at this time, the anti-reflective effect may be greatly reduced, thereby causing serious double images; (5) since the anti-reflective coating is disposed on the air-contacting surface of the laminated glass, the reflected image formed on the additional low-emissivity coating may not be eliminated, double images may still exist to some extent.
Likewise, the visible double images may also reduced by the means of enhancing the reflected light formed on the air-contacting surface of the laminated glass. For example, in a filming HUD projector common in parts markets, a semitransparent reflecting film adheres to the HUD projection area in advance; or as disclosed in U.S. Pat. No. 6,137,630A, a plurality of dielectric layers having a high refractive index and a plurality of dielectric layers having a low refractive index are deposited alternately on an air-contacting surface of a laminated glass, thus enhancing the intensity of the reflected light on the air-contacting surface, while the intensity of the reflected light on the other surface is barely changed, and therefore the visible reflected image is mainly formed by the reflected light on the surface of the film. Apparently, in this technical solution, on one hand, the uniformity and appearance of the entire front windshield of the automobile is destroyed, and on the other hand, the problem of double images may not be solved thoroughly.
In the head-up display system of the automobile, in addition to solving the problem of double images, the display system is required to be capable of displaying colors as much as possible, so as to be capable of displaying various different information images. For example, as disclosed in Chinese Patent CN2694293Y, the head-up displayer is enabled to have narrow wavelength reflection bands of the three primary colors, red, green and blue, by depositing a plurality of films having different refractive indexes on a substrate; or as disclosed in U.S. Pat. No. 6,137,630A, the plurality of dielectric layers having a high refractive index and the plurality of dielectric layers having a low refractive index are deposited alternately on the air-contacting surface of the laminated glass to achieve full-color display. Actually, if the full-color display requirement may not be met, the green light reflection to which human eyes are most sensitive should be ensured as much as possible.